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'   THE BLAST FURNACE.                                          53
carries away considerable manganese, .but if the cinder is basic most of the manganese is reduced and appears in the iron! !In the making of spiegel iron and ferro-manganese, it is necessary to have a 'strongly limey cinde'r'to" prevent waste of manganese, so that the silicon is usually low in these alloys. It is possible, however, by special care, to make a silico-spiegel with as much as 11 per cent. of silicon and 18 per cent, of manganese, this being used as a recarburizer in steel making.
SEC. Ilk.*  Chemical reactions.  A blast furnace may be; looked upon as a colossal gas producer, in which there is a column, of coke 70 ft. high ranging in temperature from a white heat at the tuyeres to a black heat at the tunnel head. As soon .as the air strikes the white-hot coke there is an immediate formation 'of carbonic acid, followed by an instantaneous reaction, by which .the caroonic acid so produced unites with more carbon to form carbonic oxide. This reaction is consummated quickly and with thoroughness, so that if the furnace held only coke,j the gas from the top would be almost entirely carbonic oxide and nitrogen ; but the furnace contains also iron oxide, and this complicates the matter, for the carbonic oxide reacts upon the, oxide of iron, forming carbonic acid and metallic iron. The reactions between carbonic acid (C02), carbonic oxide (CO), carbon, ferric oxide (Fe203), ferrous oxide (FeO) and spongy iron (Fe) are dependent upon the temperature and upon the composition of the gases. The phenomena -vf ere investigated by Bell many years ago, and Fig. II-C, as well] as the following discussion, is founded on his experiments. . ;- \
Carbonic oxide begins to reduce Fe203 at about 250o;-C! (480 IT.'), but the action is not rapid until a temperature of 400 C. to 4500 C. is reached (say 800 F.), when the Fe,03 is converged into Fe304, or after longer exposure, to Fefl07. Following are some of the chemical relations between carbonic oxide and the usilal iron oxides, in the order in which they occur-in ;the blast furnace: 
(1) 3 Fe2Q3+CO=2 Fe30+C02.                  . 
^         (2) Fes04-f CO=-3 FeO+C02.          <,,
(3) FeO+CO=Fe+C02.
Each of these is exothermic  i.e.., it produces heat.       ^ ........
* I am indebted to Mr, J, W. Dougherty superintendent of the Pennsylvania Steel "'                     'Co., at Steelton, ifor a careful supervision of this section.